1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and a fabrication method thereof. More particularly, the present invention relates to an LCD device having a high aperture ratio and a high optical transmittance that enhance a fabrication yield and reduce a number of masks required in a fabrication method. The present invention also relates to a fabrication method of the LCD device.
2. Discussion of the Related Art
Recently, flat panel display devices, such as liquid crystal displays (LCDs), plasma display panels (PDPs), field emission displays (FEDs), and vacuum fluorescent displays (VFDs), have been developed because of their small size, light weight, and power-efficient operations. Various portable electric devices, such as mobile phones, personal digital assistants (PDAs), and notebook computers have been developed to incorporate these flat panel display devices. Of these flat panel display devices, LCDs are in high demand and are currently mass produced because of their simple driving scheme and superior image quality.
An LCD device is a display device that displays a desired image. The LCD device independently supplies a data signal to pixels arranged in a matrix according to image information. Thus an optical transmittance in each of the pixels is controlled. The LCD device may be driven by an active matrix (AM) method. The AM method serves to drive liquid crystal by applying a voltage to the liquid crystal through a switching device such as a thin film transistor (TFT) provided at each pixel.
LCD devices may be classified according to a method that drives liquid crystal molecules. Such classifications include an LCD device of a twisted nematic (TN) mode and an LCD device of an in-plan switching (IPS) mode. These modes are currently used.
A TN-mode LCD device drives liquid crystal molecules in a perpendicular direction relative to a substrate by turning on/off an electric field. The presence or absence of the electric field causes the direction of the liquid crystal molecules to have an angle of about 0°-90° based on the substrate.
However, because the TN-mode LCD device drives liquid crystal molecules in a perpendicular direction relative to a substrate, a narrow viewing angle is obtained. As a result, a color or a brightness of an image is varied according to an arrangement direction or an arrangement angle of the liquid crystal molecules in the LCD device. An in-plan switching (IPS) LCD device aids in solving this problem. In an IPS-mode LCD device, a wide viewing angle is implemented. That is, an in-plan electric field is applied to a substrate in a horizontal direction. The in-plan electric field drives liquid crystal molecules such that the direction of the liquid crystal molecules is parallel to a substrate along the electric field direction.
When a voltage is applied to an electrode of the IPS-mode LCD device, a horizontal field is formed on a substrate that aligns liquid crystal molecules in a horizontal direction. Accordingly, the IPS-mode LCD device obtains a relatively wide viewing angle when compared with the TN-mode LCD device. FIG. 1 is a schematic view showing a unit pixel of an IPS-mode LCD device according to the related art.
As shown in FIG. 1, a gate line 1 and a data line 3 formed of a metal layer are arranged on a first substrate of an LCD device in horizontal and vertical directions, thereby defining a unit pixel. The pixels are implemented as n×m pixels on an LCD device as n gate lines 1 and m data lines 3 that cross one another. However, only one pixel is shown in FIG. 1.
A switching device, such as a thin film transistor (T) composed of a gate electrode 1g, a semiconductor layer (not shown), and source/drain electrodes 3a and 3b is formed at a crossing between the gate line 1 and the data line 3. The gate electrode 1g and the source/drain electrodes 3a and 3b are connected to the gate line 1 and the data line 3, respectively. The gate electrode 1g turns on the switching device by a signal input through the gate line 1, and the source/drain electrodes 3a and 3b transmit signals input through the data line 3 to the pixel.
A common line 11 that transmits a common signal is arranged in the unit pixel in parallel with the gate line 1. At least one pair of electrodes for switching liquid crystal molecules, such as a common electrode 13 and a pixel electrode 15, are arranged in the unit pixel in parallel with the data line 3, thereby generating a horizontal electric field parallel with a surface of the substrate.
The common electrode 13 may be simultaneously formed with the gate line 1, and connected to the common line 11. The pixel electrode 15 may be formed on a passivation layer (not shown) formed on an entire surface of the substrate including the source/drain electrodes 3a and 3b, and is connected to the drain electrode 3b through a contact hole 7.
A storage electrode 11′ extending from the drain electrode 3b overlaps with the common line 11. A gate insulating layer (not shown) is interposed between the storage electrode 11′ and the common line 11, thereby forming a storage capacitor.
A black matrix that prevents light from leaking into the thin film transistor, the gate line 1, the data line 3, and a color filter layer that implements colors are formed on a second substrate (not shown). An overcoat layer that planarizes the color filter layer is formed on the color filter layer.
An alignment layer (not shown) that determines an initial alignment direction of liquid crystal is formed on each facing surface of the first substrate and the second substrate, and a liquid crystal layer is formed between the first substrate and the second substrate.
In the IPS-mode LCD device, the common electrode 13 and the pixel electrode 15 are arranged on the same substrate to generate a horizontal electric field. Also, liquid crystal molecules of the liquid crystal layer are driven such that they are parallel with the substrate by the horizontal field. Therefore, an image of the LCD device may be displayed in upper and lower directions and right and left directions. That is, a wide viewing angle in upper and lower directions and right and left directions may be obtained.
However, in the related art IPS-mode LCD device, the common electrode 13 and the pixel electrode 15 of an opaque metal layer are arranged in a pixel region for displaying an image. Therefore, an aperture ratio of the LCD device is decreased, and an optical transmittance is lowered. Furthermore, a backlight having a higher intensity is required in order to achieve a desired brightness of the LCD device, thereby increasing power consumption.
In order to solve the above-mentioned problems, a method for forming a pair of electrodes with a transparent conductive material has been proposed. However, the method only slightly increases an aperture ratio and an optical transmittance was not greatly improved due to the following reasons. Liquid crystal molecules positioned at an upper region of an electrode having a wide width in order to generate a horizontal field of certain intensity are not influenced by the horizontal field. Therefore, the liquid crystal molecules maintain the initial arrangement, and thus an optical transmittance is not improved. Even if both the common electrode and the pixel electrode are formed of a transparent conductive material, the transparent conductivity influences an edge of the electrode to increase a white brightness without influencing a middle region of the electrode.